Basic Rail Testing Approaches
In the wake of several train derailments in the 1920""s, it was determined that nondestructive testing methods for locating structural flaws in railroad rail was needed. Initial work focused on an approach wherein a current was applied to the rail and the drop in voltage used to determine the presence of a discontinuity within the rail. This voltage drop technique, although successful statically, proved to be unreliable when testing was carried out using a test car moving over the rails being tested. Subsequent research focused on magnetic induction techniques.
Induction testing is based on simple physical principles. A large direct current is injected into the rail using two sets of contacts or brushes as shown in FIG. 1. Discontinuities in the railhead section cause a disturbance of the current flowing through the railhead between the contacts. The discontinuity is detected using a sensing head that responds to the accompanying magnetic field disturbance. Perturbations in the magnetic field around the railhead are detected as induced voltages in search coils in the sensing head.
Magnetic induction was the dominant rail inspection technique until the introduction of ultrasonic techniques. Initially seen as complementing magnetic induction, ultrasonics later became the dominant technique. In the typical ultrasonic inspection unit, ultrasonic transducers are installed in pliable wheels that ride over the upper surface of the rail. The pliable wheels are filled with a coupling fluid and are in contact with the rails under pressure. The transducers are arranged to send ultrasonic signals at different angles into the rail and especially the railhead. The return signals are processed and used to map the locations of flaws in the rail.
Types of Rail Defects
Rail defects can occur in the rail head, web or base. Defects are usually a result of impurities in the original ingot that were elongated during the forging process. Depending on the nature of the impurity, the resulting flaw can grow along the axis of the rail or transverse to this axis. Transverse defects may also result from service-induced anomalies, such as work hardening of the railhead. Some of the more common defect classifications are as follows:
Transverse Fissure. This type of defect is usually centrally located in the railhead and results from an oxide inclusion or other small impurity that causes a xe2x80x9cstress riserxe2x80x9d in the rail. See FIG. 2. Growth of the inclusion flaw is promoted by the constant flexing of the rail. This growth generally continues until the rail eventually fractures. A fracture of this type exhibits xe2x80x9cgrowth ringsxe2x80x9d as shown in FIG. 2.
Detail Fracture. This type of transverse defect usually occurs as a result of the work hardening of the railhead. This causes a split in the railhead and a transverse separation that typically begins on the gage side of the rail as shown in FIG. 3. (The xe2x80x9cgage sidexe2x80x9d is defined as the side of the rail along which rail car wheel flanges run.) Another mechanism for this type of rail failure is an anomaly known as a xe2x80x9cshell.xe2x80x9d A shell is usually caused by a horizontally oriented, axial, linear impurity (a xe2x80x9cstringerxe2x80x9d) that becomes elongated and flattened during use. A shell is not usually classified as a defect in itself; however, it is common for such a condition to subsequently result in a transverse defect.
Vertical Split Head. A railhead stringer that is vertically oriented can grow in the vertical plane along the axis of the rail. This is referred to as a vertical split head and is potentially an extremely serious type of defect as it can result in the loss of the running surface of the rail. See FIG. 4. A horizontal split head usually originates from a longitudinal seam or inclusion. Growth usually occurs rapidly along the length of the inclusion and spreads horizontally as shown in FIG. 5.
Head and Web Separation. This type of defect is usually found at the end of the rail (i.e., at a joint). Such separation is believed to occur due to eccentric loading at the end of the rail. The separation occurs at the weakest point, which is where the railhead joins the web at the fillet. FIG. 6 shows a head and web defect that has progressed into the fillet area.
Bolt Hole Cracks. These defects are usually as the result of stresses applied to the edge of a bolt hole by the bolt. Such stresses are produced due to the cycling up and down of the joint as a train passes over it. The effect may be worsened by worn joint bars or improper drilling. A severe case is shown in FIG. 7.
Engine Burn Fractures. These defects result from wheel slippage during acceleration of a locomotive from a standstill. Rapid heating and cooling causes thermal cracks that are exacerbated by the train wheels pounding the area. Transverse separation can occur as a result. An example is shown in FIG. 8.
Defective Welds. Weld defects vary according to the weld type. in general, there are welds that are made during rail manufacture and there are welds that are made on site while the rail is being installed or repaired. Manufacturing welds are usually xe2x80x9cflash buttxe2x80x9d welds. Welds made in the field are mostly xe2x80x9cthermitexe2x80x9d welds. Defects that are germane to the flash butt type of weld are for the most part fusion type flaws. Thermite welding is actually a type of casting operation where a mold is situated around the profile of the rail and molten metal is allowed to flow between the mating surfaces. The flaw possibilities from a thermite weld can be more diverse, ranging from lack of fusion to porosity or other non-metallic inclusions.
Statistically, defects and associated failures can be broken down as follows:
Factors in Flaw Detection
Defect detection in railroad rails is complicated by the fact that rails come in a variety of shapes and sizes. The accessible scanning surface, which is usually the railhead, is extremely non-uniform. In addition to variability of the rail as manufactured, bead shape changes over time as a result of use by high speed, high axle-load trains. The resulting non-uniformity of the rail geometry renders it difficult to maintain the contact of sensor equipment with the rail head. The difficulty is exacerbated by curves, crossings and switches. In addition to affecting data, these track components can be hazardous to the sensor equipment that contacts the rail.
The surface condition of the railhead can be an important limitation on sensor sensitivity. A railhead having rust, grease or other foreign matter such as leaves on its surface can severely inhibit the transfer of energy from an ultrasonic transducer mounted within a rail search unit tire. Search unit tires may also be punctured by steel slivers that develop on the railhead surface.
Weather can be a significant factor in flaw propagation. Contraction of the rail due to cold temperatures combined with heavy train axle loads are very conducive to flaw separation, particularly when a train has a flat spot on a wheel that happens to contact the rail at a critical location relative to the flaw. Weather can also have a significant impact on flaw detection. Formation of ice in particular can make testing extremely difficult.
Regardless of the system quality or its ability to detect defects, personnel and their training are an integral part of the equation. Experience has shown that proper personnel selection, combined with a good training and certification program usually leads to well qualified personnel in the field. Experienced personnel are able to add to the effectiveness of the system through their ability to note anomalies by simply watching the track as it is tested.
Not all rail defects are detectable by either the magnetic induction technique or the ultrasonic technique. Using a combination of the two methods greatly reduces the number of xe2x80x9cfalse callsxe2x80x9d (i.e., indications of a defect where such an indication is actually unwarranted).
Accordingly, it is highly desirable to conduct defect testing using both magnetic induction and ultrasonics as complementary methods. Heretofore, this has required a large rail-bound test vehicle that houses both ultrasonic and magnetic induction equipment and its associated data acquisition and processing equipment. Hi-rail inspection vehicles currently use only ultrasonic detection systems because, heretofore, the equipment required to generate the power for magnetic induction testing has been too large for such a vehicle. The railroads have therefore been prevented from taking full advantage of combined ultrasonic and induction testing.
An embodiment of the present invention accordingly provides a railroad rail inspection system for use in conjunction with a non-railbound vehicle having an equipment bay. The system comprises a detector carriage adapted for being propelled over a two-rail railroad track by the non-railbound vehicle. A magnetic induction sensor system is attached to the detector carriage. The magnetic inductor sensor system is adapted for magnetic induction inspection of at least one rail of the track. The system further comprises a data acquisition system in communication with the magnetic induction sensor system. The data acquisition system includes at least one data processor adapted for processing induction data received from the magnetic induction sensor system. The system still further comprises a power supply system in electrical communication with the magnetic induction sensor system. The power supply system is adapted for supplying electrical power to the magnetic induction sensor system. The data acquisition system and the power supply system are configured for disposition and operation within the equipment bay of the non-railbound vehicle.
Another aspect of the invention provides a railroad rail inspection system for use in conjunction with a non-railbound vehicle having an equipment bay in which the system comprises a detector carriage adapted for being propelled over a two-rail railroad track by the non-railbound vehicle. The system further comprises means for performing magnetic induction inspection of at least one rail of the track, the means for performing magnetic induction inspection being attached to the detector carriage. The system further comprises means for processing induction data received from the means for performing magnetic induction inspection and means for supplying electrical power to the means for performing magnetic induction inspection. The means for supplying electrical power includes means for generating power sufficient to establish a magnetic field around the rail for use by the means for performing magnetic induction inspection. The means for processing induction data and the means for supplying electrical power are configured for disposition and operation within the equipment bay of the non-railbound vehicle.